Pavel Sebor himself has said they plan to support ETS2 for five more years. That was February 2016, so that means we're just over 3.25 years away from finding out if ETS2 will continue or be replaced with ETS3. No one (important) has ever said ETS3 will not happen. Just that it won't happen yet.
It will really come down to what is best business wise. An ETS3 offers SCS far more good points than bad, right off the bat it means they can do what they have wanted to do for some time, completely ditch legacy (retail) versions and 32bit support.
The whole of the old base map can be rebuilt, which they can charge for, they can incorporate all the options they need to to allow future DLC to DLC dependency by-passes.
The downside would be trucks. Would they be able to renegotiate all the licenses? Scania and DAF would be easy, but the others might not be and there is a huge risk that we could lose the older models as well.
Fortunately SCS use their own engine, so they have options. The question isn't whether ETS3 will/won't happen, it's which option is the most profitable long term and I don't think any of us on the outside can even begin to cover all the possibilities to reach such a decision.